BAGHDAD, Iraq (CNN) -- The U.S. military virtually abandoned an area southeast of Baghdad known as Salman Pak two years ago, leaving behind a refuge for insurgents to operate and allowing them to establish a key explosives smuggling route to Baghdad.

U.S. forces have returned to Salman Pak, an area where insurgents have established a stronghold.

Now, with 28,000 additional troops deployed to Iraq under the military's so-called "surge," U.S. troops are back in Salman Pak in force. They are working with Iraqi security forces to retake lost ground, break up insurgent supply lines and clear the area of al Qaeda fighters and insurgents.

But their presence demonstrates a problem for U.S. forces trying to secure Baghdad and its perimeter:Iraqi security forces are not yet able to secure and hold areas on their own.

"Do I think the Iraqi security forces are improving? Yes," said U.S. Army Col. Wayne Grigsby of the Third Infantry Division. "Are they where they need to be? No."

With Congress intensifying its calls for an exit strategy for Iraq, a report released last week by a subcommittee of the House Armed Services Committee said the United States has spent $19 billion training and equipping 346,500 members of the Iraqi security forces. That $19 billion has "yielded mixed results," according to the report.

The report found that the U.S. Defense Department doesn't know how many of those forces are operational today or even if they are fighting on the U.S. side. "Of those forces trained by the coalition, there is strong evidence that some are independently committing sectarian violence and other illegal activity," the 205-page report said.

The Iraqi forces, the report adds, are "not yet ready to take full responsibility for their nation's security."

Last Thursday, President Bush defended his administration's surge policy and urged patience on Iraq. "This new strategy is different from the one we were pursuing before," Bush said in a speech to the U.S. Naval War College. "It recognizes that our top priority must be to help the Iraqi government and its security forces protect their population from attack, especially in Baghdad."

Short of manpower, U.S. forces left much of the area two years ago, leaving only a few hundred troops for the whole region. The streets of Salman Pak, once a resort area for members of Saddam Hussein's regime, are littered with empty bottles and plastic bags, and many of the buildings have been damaged or destroyed.

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As a convoy of U.S. Humvees moved cautiously through the area this week, Grigsby pointed out holes in the ground along the side of the road -- places where U.S. military vehicles have been targeted by roadside bombs.

Some of the bombs were planted less than 100 yards away from Iraqi police checkpoints. "Some of the Iraqi police are complacent when insurgents place bombs," Grigsby says. "They have gotten better, but in many cases we can't really trust them yet."

Farther down the road, U.S. soldiers battled insurgents, cleared the area and then set up a checkpoint at a major road along the Tigris River that leads into Baghdad. After the three days of fighting, U.S. forces turned the checkpoint over to the Iraqi National Police.

At the time, the Iraqi commander, Col.Shakr, who would not give his full name due to security concerns, told CNN he hoped his forces would be able to hold the outpost, with its two battle tanks, strong weaponry and a guard tower overlooking the entire area.

But just two hours later, the insurgents struck in broad daylight from a nearby mosque. The attackers engulfed the checkpoint in a barrage of small-arms fire and rocket-propelled grenades, taking out the guard tower and killing several Iraqi officers.

Shakr called in coalition reinforcements in the form of an airstrike and moments later, a British Tornado strike aircraft dropped a 2,000-pound bomb on the insurgents, obliterating the building they were in.

Grigsby directed the airstrike. He said he was pleased with the results, especially because U.S. forces rarely see insurgents mass in a single area that allows coalition forces to use their bombs effectively.

But the incident also demonstrates just how much coalition forces are still needed -- that American forces are often called back to places they secured only hours before because their Iraqi counterparts are not up to the task.

The House panel's report said Iraqi forces may not reach full operational capability until early 2009. Even then, it says, "the Iraqis will still require trainers and advisers, as well as critical combat enablers including logistics and intelligence support."

U.S. military leaders in Iraq say they are determined to see the surge through.

"There has to be persistent security presence, and that has to be Iraqi security forces. So we continue to work with the government of Iraq and the leaders of the Iraqi security forces to get that persistent presence," said Army Maj. Gen. Rick Lynch.E-mail to a friend